In recent years, there has been a request to form an air gap between copper (Cu) interconnects. For example, the air gap can be formed by forming the Cu interconnects in a sacrificial film, removing the sacrificial film after forming the Cu interconnects, and forming an insulator having a poor filling property on the Cu interconnects after removing the sacrificial film. However, the Cu interconnects in this case may dissolve or disappear when removing the sacrificial film by wet etching or the like. A similar problem may occur in a case of forming an air gap between interconnects other than the Cu interconnects.